In developing a process for dry deinking of wastepaper by fiberization of the wastepaper and separation of the fibers from the ink-containing fines, it was found preferable to first shred the raw wastepaper feed into more manageable pieces prior to fiberization. For reasons of quality control and also because of a potential fire hazard within the fiberizer due to the presence of airborne cellulosic dust, it is necessary to remove all pieces of metal, such as staples, paper clips, etc., which are present in the raw wastepaper feed and which might spark a fire if fed into the fiberization apparatus. It is of course also desirable to remove any other undesirable non-paper materials such as glass, wood, stones, etc. Therefore a means for removing foreign matter from the shredded paper needed to be developed.
Prior art methods for separating dense materials from lighter materials are well known. For example, U.S. Pat. No. 3,907,670 to Fernandes teaches an apparatus for classifying a mixture of particulate solids as found in municipal refuse. The mixture of materials is blown through a slightly downwardly declined duct, at the end of which is a vertical duct which collects heavy materials. The lighter materials are airborne through an upwardly inclined duct. Any heavy materials which passed over the vertical duct (46) cannot be carried by the air up the incline and fall back down into the vertical duct.
U.S. Pat. No. 3,986,949 to Di Duca et al. and U.S. Pat No. 3,441,131 to Gebauer teach a similar method wherein forced air suspends and carries lighter materials in an upward direction while heavier materials, unable to be sustained by the air flow, drop into receptacles.
However, the prior art designs are not believed to be suitable for consistently removing relatively small particles such as paper staples, grains of sand, etc. from the shredded paper. This is due to the critical nature of the air velocity required to convey paper and yet permit the small particles to drop out. It is believed that such a critical air velocity could not be consistently maintained in the commercially available separators due to factors such as fluctuation in paper feed rates and the effect of turbulence on air flow uniformity. Therefore there is a need for an improved separation method to remove foreign matter from shredded paper.